The thermal conductivity of thermal insulating material is a primary characteristic to be taken into consideration when designing and comparing insulation systems. Thus it is clearly wasteful to use more of an expensive insulating material than is actually necessary to obtain a required level of insulation, but it is also vital to ensure that sufficient insulating material is provided to achieve that level. The calculation of the appropriate quantity of insulating material involves knowledge of the thermal conduction characteristics of the insulation system. This parameter is dependent both on the inherent thermal conductivity properties of the material and on the environmental and geometric conditions in which it is to be used. Thus the actual temperatures and shapes of the thermal insulating components must be taken into account.
This can lead to difficulties in the design of efficient and cost-effective insulating systems for pipes, for example. The thermal conductivity of insulating material for use with pipes can in principle be measured on a standard flat plate tester. In such a device a heater plate which is typically square is surrounded at its periphery by a guard ring which prevents heat loss from the edge of the plate. Flat sheets or panels of the thermal insulation material to be tested are placed on both faces of the plate and extend to cover the guard ring. The plate, which defines the test section, is heated and the guard ring is also heated to match the plate temperature. The thermal conductivity of the insulation is calculated from measurements of the temperatures on its hot and cold faces and of the heat flowing through it. However, the conductivity value obtained is not necessarily representative of the behaviour of the insulating material when installed around a pipe. If the insulating material is fibrous, for example, the orientation of the fibres can affect the conductivity. Thus any difference in the fibre orientation as between the tester and the installed insulation may result in the measured conductivity being unrepresentative of the actual insulation system. Furthermore, the effective conductivity of the entire installed system is affected by factors such as heat transfer at butt joints in the insulation or at gaps which are created by thermal expansion, and the effect of any supporting and protective structure such as a metal casing. The effect of these factors is not included in the value obtained with a flat plate tester.
In view of these limitations, it is preferable to test pipe insulation in a configuration more closely resembling its actual conditions of use. In order to measure the thermal conductivity of insulating material applied to a pipe it is necessary to know the temperature difference across the insulation, the quantity of heat passing through a unit length of the insulation and the physical dimensions of the insulation. The conductivity L is given by EQU =(Qxln(D/d))/(2xpix(t-T))
where
Q is the heat flow per unit length of the pipe; PA1 D is the outer diameter of the insulation; PA1 d is the inner diameter of the insulation; PA1 pi =3.1415O26; PA1 t is the temperature at the inner surface of the insulating material; and PA1 T is the temperature at the outer surface of the insulating material.
Two alternative procedures have been developed in the past for measuring conductivity values of pipe insulation. However, both have their own disadvantages. In a first method a straight section of insulated pipe is heated by means of electric heating elements wound around it, inside the insulating material. Thermocouples are attached to the pipe, in particular at two locations intermediate the mid-point of the pipe and each end. The central section of pipe, between these intermediate locations, is considered as being the actual test section and the end sections are treated as guard sections to compensate for end effects. Each section is provided with its own heating element or elements. The electric power supplied to the heating elements is adjusted for each section individually until a steady state is attained with no temperature gradient at the boundaries between the ends of the test section and the adjacent guard sections. It is assumed that in this state there are no losses of heat through the ends of the test section, so that all the heat supplied by that section's heating elements is traversing the insulating material around that section. Thus the heat loss per unit area of the pipe surface can be obtained and a value of thermal conductivity derived. However, this technique assumes that there is no heat transfer between the test section and the guard sections either through the pipe wall or along the interior of the pipe. In practice this requires very elaborate and cumbersome arrangements, and even then not all heat leakage may be eliminated, leading to inaccuracies in the result. Furthermore, establishing the required condition of no temperature gradient can be difficult and time-consuming.
A second method involves two straight pipes, one of which is longer than the other. The longer pipe is considered as having a central test section between guard sections each of which is half the length of the shorter pipe. Typically the pipes might be three meters and one meter long, giving a test section of two meters between half-meter guard sections. The thermal behaviour of the two halves of the shorter pipe is assumed to be identical to that of the guard sections in the longer pipe. Each pipe is fitted with heating elements, thermocouples and insulation in a similar manner to the first technique. Both pipes are heated to a range of temperatures and the electric heating energy supplied to obtain a steady state is plotted against the average pipe temperature. The difference in heat energy supplied to the two pipes is attributed to the heat loss from the centre test section of the longer pipe. In practice it is very difficult or impossible to create conditions in the short pipe that are identical to those in the guard sections in the longer pipe, and the results are correspondingly inaccurate.
It is an object of this invention to provide a method of testing the thermal conductivity of non-planar insulating material such as pipe insulation, which avoids or alleviates the problems and inaccuracies encountered with known test methods. It is also an object of the invention to provide an apparatus for performing the method.